ZnS and ZnSe semiconductors have band gaps of approximately 3.7 eV and 2.7 eV respectively where attention has been given to them as so-called wide gap semiconductors with being developed vigorously as optoelectronics light emitting materials having colors ranging from blue to green (a compound semiconductor containing the ZnS and ZnSe semiconductors as main components will be hereinafter referred to as a ZnS type semiconductor).
The semiconductors of this type with an n type can be fabricated relatively easily, however, it is hard to have a low resistance with a p type. For this reason, various technical developments have been attempted. A technique using nitrogen (N) as a dopant has been utilized for causing ZnSe to have the p type. As described in Patent Document 1, this is a technique for simultaneously irradiating a nitrogen (N) radical to a film and introducing N into the lattice to obtain p-ZnSe during the formation of said film through an epitaxial growth using an MBE (molecular beam epitaxy) method in which a single crystal of ZnSe or GaAs is employed as a substrate. The MBE method is a film forming technique in which a high degree of vacuum is indispensable, and a radical source controlled precisely is indispensable to the efficient generation of the N radical. For these reasons, there has been an industrial problem in its productivity Moreover, it has been reported that the upper limit of a carrier concentration of ZnSe based on N is 1018 cm−3 (Non-Patent Document 1). Therefore, a resistivity is somehow still insufficient for an application to a unit, and a material having a lower resistance and an excellent p-type semiconductor characteristic has been desired under present conditions.
Moreover, there is a problem in that it is impossible to form an ohmic contact by using an electrode metal because the carrier concentration of the p-ZnSe is not sufficiently high under the present conditions. For this reason, conventionally, there has been employed a technique for forming a multiquantum well (MQW) structure using ZnTe which can easily form a p type and a low resistance between an electrode metal and p-ZnSe. However, there is another problem in that a lattice defect is increased during an operation by the generation of Joule heat in this structure and an electrical resistance is thus increased to cause a deterioration in a unit (Non-Patent Document 2). Since the ZnTe is a substance having a band gap of 2.3 eV, therefore, it has a great light absorption for a light having a wavelength of 540 nm or less. For this reason, there is a problem in that a high extraction efficiency (light emission efficiency) cannot be expected in a light emitting device for emitting a light in a shorter wavelength than the wavelength region.
ZnS is a material which has a lower valence band top energy than ZnSe and is harder to have a p-type low resistance than ZnSe. A stable fabricating technique for obtaining p-ZnS has not been developed yet. For this reason, a resistivity can be reduced to 102 Ωcm at most and is entirely insufficient for a unit application under the present conditions.
Conventionally, it has been reported that deep acceptor levels of approximately 1250 meV and 650 meV are formed respectively when Cu is doped to ZnS and ZnSe (Non-Patent Document 3). It has not been scrutinized yet that these materials are caused to have a low resistance by the dopant of Cu. A thermal energy at a room temperature is approximately 26 meV. For this reason, the activation of a carrier from such a great level cannot be expected at all.
In the situations described above, a Cu dopant concentration being conventionally scrutinized is 1019 cm−3 (corresponding to 0.05 at % or α=approximately 0.001) at most. It has been known that Cu forms a deep acceptor level in ZnS and ZnSe as described above. An activation ratio of a carrier estimated from the acceptor level position (a ratio of a carrier concentration to an doped Cu concentration) is 10−5 at most or less than that at a room temperature. For example, a carrier concentration obtained in the case in which Cu is doped in a concentration of 1019 cm−3 (corresponding to 0.05 at % or α=approximately 0.001) is 1014 cm−3, and a carrier concentration which is obtained is estimated to be 1016 cm−3 at most even if Cu is doped in a concentration of 1021 cm3 (corresponding to 5 at % or α=approximately 0.1). Therefore, it has been supposed that a practical electrical characteristic cannot be obtained at all.
In recent years, moreover, the formation of a unit on a glass substrate or a plastic substrate has been an important issue in the field of a semiconductor light emitting device. Correspondingly, the application of an amorphous phase or a polycrystalline phase has also been required for an electrode layer and an active layer. However, a conventional ZnS based p-type semiconductor material has been indispensable to be a single crystal material or an epitaxial growth film. In particular, a single crystal of ZnS or ZnSe has been used as a homoepitaxial substrate, and a single crystal of GaAs or Si having a similar crystal structure and a small difference in a lattice constant has been used as a heteroepitaxial substrate. A high temperature of 300° C. or more has been required for the temperature of a substrate in the formation of these epitaxial growth films. These are requirements for fabricating an epitaxial film having few defects and high quality. More specifically, the characteristic of a p-type semiconductor has been first obtained for a single crystal having few defects and high quality. For this reason, it is necessary to raise a film forming temperature. On the other hand, in the case in which these materials are set to be a polycrystalline phase or an amorphous phase, a lattice defect generated in a grain boundary or a lattice defect induced to a lattice strain is introduced. As a result, there is caused a hindrance that a carrier is trapped into these lattice defects. For this reason, the activation of a dopant is suppressed so that it is impossible to obtain a low resistance material which is excellent. In such a situation, there has not been made a trial of utilizing the ZnS based p-type semiconductor material in a configuration other than the single crystal or the epitaxial film.
Patent Document 1: JP Patent No. 3078611 (Patentee Minnesota Mining & Manufacturing, University of Florida), also published as JP 4-234136.
Non-Patent Document 1: Journal of Crystal Growth, vol. 197(1999), pp. 557-564/W. Faschinger.
Non-Patent Document 2: Journal of Crystal Growth, vol. 214/215(2000), pp. 1064-70/A. Katayama et al.
Non-Patent Document 3: Optophysical Property Handbook (Asakura Bookstore, 1984) pp. 182-185.